Without limiting the scope of the invention, its background is described in connection with the etching of CaF.sub.2 epitaxial layers for resonant tunneling devices, as an example.
Heretofore, in this field, resonant tunneling devices have been made using GaAs/AlGaAs structures. However, if a practical resonant tunneling technology for the silicon analog could be devised, such a technology could leverage the existing mature silicon processing technologies and device product lines.
A silicon-based resonant tunneling technology would require a material that can be epitaxially interposed between two silicon layers. The interposed, or sandwiched, epitaxially formed material must exhibit a bandgap larger than silicon. It is possible to utilize the CaF.sub.2 /Si interface to form resonant tunneling devices because CaF.sub.2 has an energy bandgap of 12.5 eV. Moreover, CaF.sub.2 has a room temperature lattice constant that is nearly identical to that of Si, and also has the same crystal space group as Si. Recent studies have indicated that the conduction band of Si is raised by 2.5 Ev, suggesting a CaF.sub.2 /Si heterostructure is a viable material for resonant tunneling. FIG. 1 shows the effect of the 2.5 Ev offset on the bandgap of CaF.sub.2.
CaF.sub.2 must be patterned along with Si to form the above described resonant tunneling devices, however no acceptable method of etching CaF.sub.2 has yet been developed. There are three methods at present to etch CaF.sub.2 : water, dilute nitric acid (HNO.sub.3) and dry etching with various energy sources. The water etch is very slow (the solubility of CaF.sub.2 in water is quite low -0.002 grams in 100 grams of water at 25 C) as well as unreliable--it is difficult to determine when etching begins, when it finishes, or even when it is ongoing. Additionally, when water is used to etch CaF.sub.2, it cracks the CaF.sub.2 film, creating "ice floe" regions, and ruining the sample. The nitric acid approach is not much better than water. Undiluted, it will not etch CaF.sub.2. Too much dilution results in the same outcome observed with the water etch, presumably associated with the higher water content. A 3:1 or 2:1 water:nitric acid dilution etches CaF.sub.2 very rapidly -2-3 seconds for a 1000-2000 Angstrom film. However, the reproducibility of this etch is very poor; the etch rate cannot be consistently reproduced in large volume production. Moreover, severe undercut of the film is a significant problem with a rapid, isotropic wet etch. This is particularly important in the use of CaF.sub.2 in resonant tunneling devices, which will require nanometer size structures.
A dry etch method driven by any of a variety of energy sources such as RF, microwave, ECR, etc. would introduce severe damage into the delicate CaF.sub.2 crystalline structure, destroying any possibility of forming nanometer scale resonant tunneling devices.
A typical problem experienced with the above methods is that the CaF.sub.2 will resist etching for a period of time and then start to etch along preferred regions very rapidly, causing the CaF.sub.2 to lift off in flakes. This results in a residual surface that is well textured. Two types of texturing are most prevalent with the prior art etch methods: undulations on the surface reflective of thickness variations and cracking of the film. Undulation-type texturing is a disaster, since small variations in the epitaxial film thickness cause large changes in the material/device behavior which are peculiar to resonant tunneling components. Cracking of the film is equally unacceptable, since leakage through the film destroys any type of device performance. Also, resistivity is an important parameter of the material that must be carefully controlled--a crack in the film would undermine the fabricated value. Accordingly, improvements which overcome any or all of these problems are presently desirable.